Summary

Several key areas relating to the performance and fault monitoring of VLANs are as follows:

• Logical ports versus physical ports

• VLAN Utilization based on broadcast and multicast traffic

• Running spanning tree within VLANs

The logical ports calculation addresses the hardware limitation of the supervisor cards installed in the switches. The formula is documented in the Catalyst Series release notes available on CCO and is reiterated here:

Monitoring broadcasts and multicasts to determine the VLAN utilization versus monitoring unicast traffic through the switches is important. Because the purpose of VLANs is to isolate your broadcast domain, it makes sense to monitor the broadcast traffic to get an understanding about how the VLAN is performing. Here is the formula we came up with for calculating VLAN utilization:

Keeping track of spanning tree changes in multiple VLAN environments is tough to do in switched networks. Rely mainly on the SNMP bridge traps to point out your next step in managing the VLAN properly and assist in identifying the changes.